Various designs of thermal imaging cameras are offered by a number of manufacturers. A “thermal image” is defined, in general, as an image produced by the optical imaging of infrared or thermal radiation.
A thermal image in the near infrared range can be produced technically by the technologies commonly employed for visible light, such as CCD or CMOS sensors. A thermal image in the middle and far infrared ranges is produced by “microbolometers,” which are arranged as a matrix of temperature-sensitive resistors and furnish in their entirety a thermal image when the signal inhomogeneities thereof, which are caused by manufacturing tolerances and by self-heating produced by a bias voltage, are corrected.
Thermal imaging cameras consisting of microbolometer components usually contain a pixel array, whose microbolometers permit a temperature-dependent current intensity based on the bias voltage (bias). Microbolometer components arranged in a matrix-like manner are usually also called FPA (Focal Plane Array).
If such a temperature-sensitive detector array is exposed via an infrared optical system, thermal images can be recorded and temperature images can thus be produced.
The following explanations are based on the following terminology for the clear understanding of the technical background.
The image is recorded in so-called “frames.” A frame is defined as the data set of the pixel intensity values of the matrix-like detector array (FPA), which are recorded for an image simultaneously. This means that a frame comes into being usually as a result of an individual reading of the entire matrix array. The reading of the image elements arranged in a matrix-like manner is usually performed row by row, and each row is in turn read subsequently column by column. A frame is now the matrix of the pixels (pixel intensities) of a complete reading process. The frame is synonymously also called the currently read image if the entirety of the metrically ordered, read pixel intensities is meant.
“Pixel-based” is defined such that the thermal image is discretized with a certain number and array of positions at which radiation intensities are measured. This is usually a row-column matrix structure, but is not limited to such Cartesian arrays. The processes described here do not presuppose a matrix structure, but also remain valid in case of another array of the pixels. The only thing that is important is that the array is the same when different images are to be compared with one another, for example, thermal image and underground image in case of an underground image correction or thermal images with different thermal sensitivities. Pixel-based means here that a corresponding correction or processing calculation is performed for each pixel position. Parameters such as sensitivity characteristics, etc., can then vary, in the most general case, from one pixel position to another, i.e., they do not have to be equal for the entire thermal image.
The mathematical definition of the terms “(strictly) monotonically rising/falling,” “reversibly unambiguous function,” “linear/nonlinear function,” “(strictly) concave/convex function,” etc., is required for the description of function curves of pixel intensities, for both processed, corrected and averaged or weighted signal values.
The term “weighting function” is defined as a set of factors, which is applied to an image data set (frame) as a weighting before the frame thus weighted is subjected to further processing. “Percentage weighting function” is defined here such that the weights shall always be between 0 and 1.
“Pixel intensity” is defined as an intensity value that is assigned to an individual pixel position of the detector matrix. Contrary to this, “image intensity” is defined as intensity data, which are assigned to the image as a whole. There are different possibilities of embodiment for both. How the pixel intensity of the individual microbolometers is usually determined will be described below. If information on the image intensity is needed as a whole, it is possible to form for this, for example, the sum of all pixel intensities. However, other definitions (e.g., mean value of all pixel intensities, sum or mean value of the pixel intensities of one subframe, i.e., of a partial matrix of the entire array or of an ROI=“Region of Interest,” i.e., of a contiguous partial area of any desired shape of the entire array, etc.) are possible as well.
When using thermal imaging cameras, the temperature differences, which are simultaneously present in a scenery to be recorded by the thermal imaging camera, are often too great to be able to be converted by the dynamic range of the electronic system (approximately linear dynamic level control range of the electronic processing chain used to record, analyze and display the thermal images) into thermal images adequately, i.e., without distortions due to saturation or noise, and they are mostly also too great to be able to be meaningfully interpreted by the user.
One example of this is the use of thermal imaging cameras by firefighters. Both the high temperatures of the source of fire and the low temperatures of humans to be rescued in the area of the fire shall be detected here in the image simultaneously in case of a firefighting mission, so that all the information that makes fast, purposeful action possible can be determined from the thermal image displayed at a glance.
The microbolometer components themselves can be actuated selectively for different sensitivities by controlling the bias voltage or integration time of the individual microbolometers or the transimpedance amplifiers thereof, connected as an integrator, doing so over a very broad temperature range. However, the difficulties lie not primarily in the sensitivity range of the microbolometer components, but in the selection of suitable temperature ranges and a suitable display. It is necessary to avoid both an overmodulation of the images and a reduction of the extent to which all details are encompassed at low temperatures due to small signals being distorted by the noise of the individual microbolometers because the (strictly monotonically rising) dynamic range is tuned to high temperatures.
The efforts are therefore aimed at alternatingly generating two thermograms each, recorded with higher and lower sensitivity and displaying them together in a suitable manner on a display in order to obtain a non-overmodulated image encompassing all details at both high and low temperatures.
To distinguish the two thermal images, which were recorded with markedly different sensitivities of the FPA, the terms “more sensitive image” and “less sensitive image” will be used below, and one also speaks of “images of medium sensitivity” in case of more than two images that are recorded with different detector sensitivity settings.
It is already known in the state of the art from GB 2 435 974 A that two alternatingly recorded thermograms can be displayed in a common image. However, the thermograms recorded there alternatingly with higher and lower dynamics are buffered individually and the two images are then sent to a common display. This procedure has the drawback that it is highly memory-intensive and requires additional memory bandwidth. Furthermore, the buffering delays the image sequence compared with a direct signal processing by one or more frames. The so-called pipeline delay increases, which is extremely undesirable for real-time applications.
A general method for the use of more sensitive and less sensitive pixels is described, for example, in JP 2004-222183 A, wherein the camera, which is equipped with more sensitive and less sensitive pixels, has an input unit for an exposure correction value, on the basis of which the images of different sensitivities are taken into account with an exposure value and fitted together into a combined image and which is used to expand or restrict the dynamic range.
A thermal imaging system, which is known from US 2007/0211157 A1, operates on a very similar basis. Repeated switchings between two sensitivity settings of the detector array are likewise performed in this solution, and the suitable sensitivity level is selected pixel by pixel with the use of a threshold value criterion and displayed.
It is disadvantageous in both cases that a quasi-simultaneous parallel processing of the two images recorded with different sensitivities must be performed, and the combination of the image data pixel by pixel depends on arbitrarily set or empirically found correction values or threshold values, which must either be adapted continually to the current thermal scenery or preselected on the basis of certain criteria.